on Wed Aug 27 2008, "Niels Dekker - mail address until 2008-12-31" <nd_mail_address_valid_until_2008-12-31-AT-xs4all.nl> wrote:
David Abrahams wrote:
What was wrong with my proposal?
You can't guarantee it will be right every time, but it's easy to correct when wrong, and that would only take a very few specializations.
Please note: The question is /not/ whether T has a custom swap. The question is whether default-construction + swap outperforms copy-construction for type T. Only in that case, it makes sense to activate "Swaptimization".
Of course. But can you come up with a type where it makes sense to have written a custom swap but default-construction + swap won't outperform copy construction? Maybe my imagination is failing me, but I can't.
Anyway, I agree, your approach provides a reasonable first guess:
std is an associated namespace of T ? has_member_swap<T> : has_nonmember_swap<T>
So for such a type T, _Move_operation_category<T>::_Move_cat should be set to "_Swap_move_tag", instead of "_Undefined_move_tag", by default. Do you know how we can technically overrule the default _Move_operation_category<T> provided by MSVC's STL?
Specialization?
In ConceptC++, it would look a little bit like this:
template <class T> where HasCustomSwap<T> // Pseudo ConceptC++ struct _Move_operation_category<T> { typedef _Swap_move_tag _Move_cat; };
But how to do that in C++03?
Ohhh... If they didn't give us any further hooks (such as a default argument) to hang a specialization on, then sadly I don't know of a way. -- Dave Abrahams BoostPro Computing http://www.boostpro.com